1. Field of the Invention
This invention relates to a needle assembly that allows simultaneous injection and exhaust of fluid for injection moulding, and in particular the injection and exhaust of a gas.
2. Background Art
In conventional gas assisted gas injection moulding methods, a pre-filling of melt material is first introduced into a mould. A gas is then injected within the melt so creating a hollow or void within the melt that exerts pressure throughout the cooling of the melt to compensate for volumetric shrinkage. Typically, a first needle assembly is used to inject gas and when injection is complete gas injection is ceased and gas is exhausted through a second needle assembly; this exhaust occurring after the melt has solidified. Hence, the injection and exhaust steps do not overlap.